An electrically operated parking brake is known, for example, from DE 10 361 042 B3, which is incorporated by reference. The latter has an electric motor which interacts with a brake piston. In addition, a gear mechanism with a high speed-reducing ratio is provided, which gear mechanism serves to convert the rotational movement of the electric motor into a longitudinal movement of the brake piston in order to activate the brake shoes (friction linings).
The document shows that, in particular in vehicle brakes without force sensors, it is assumed that a predetermined brake-application force is reached (and the electric motor can be switched off) if the power drain of the electric motor has reached a predetermined value, the switch-off current value or maximum current value. In the known method, in order to bring about a predetermined brake-application force, the size of a switch-off current of the electric motor is additionally varied as a function of an event number of a selected event, for example the activation of the brake, the stopping of the vehicle, the locking of the vehicle and/or other such events. Further correction factors may take into account further sensed characteristic variables such as temperature and resistance of the electric motor in the adaptation of the switch-off current.
Document DE 19 732 168 C2, which is incorporated by reference, also describes a method for operating a parking brake in which, during the application of the brake, a spindle is driven by means of an electric motor until a previously defined maximum value of the current drain, corresponding to a desired brake-application force, is reached. As soon as this value is reached, the electric motor is switched off and the vehicle brake carries out its parking brake function in particular by virtue of the fact that the gear mechanism is self-locking.
Document EP 0 478 642 B1, which is incorporated by reference, also, describes an activation device for a motor vehicle parking brake in which an electric motor which is embodied as a direct-current motor and whose running direction can be switched over is used. The open-loop and closed-loop control device which controls the parking brake as a function of operating parameters of the motor vehicle and/or of the electric motor can switch off the electric motor automatically if the motor current exceeds a predetermined value or if it no longer rotates for a specific period of time.
An electric parking brake has the fundamental task of reliably arresting a motor vehicle by means of electrically driven friction brakes in accordance with the legal and/or customer-specific requirements. The driver of a motor vehicle with such an electric parking brake must be able to count on the functional reliability of this brake-by-wire brake system in this context.
A central customer requirement prescribes that at least a predefined brake-application force (for example 18.5 kN) is to be set whenever the brake is applied, regardless of the environmental influences or other peripheral conditions. This requirement already takes into account all the vehicle-specific and manufacturer-specific actuators, in particular the maximum expected positive pitch, the diameter of the brake discs, the coefficients of friction of the linings etc. It is also necessary to ensure that when the electric parking brake is released, a sufficiently large brake-application clearance is set so that the brake does not overheat during the further operation of the motor vehicle.
A basic problem of contemporary open-loop/closed-loop control systems of parking brakes is that the brake-application force is not measured directly, for reasons of cost, but instead is determined indirectly from other parameters. For example, known methods for applying the parking brake use, as stated above, a previously defined maximum current or switch-off current as a switch-off criterion, for which it is assumed that the desired brake-application force is reached. However, as is recognized nowadays, the known methods do not operate precisely enough, with the result that the brake-by-wire brake system frequently does not exhibit the required level of reliability.